Iñaki Mondragon

Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites

Abstract Chemical treatment of natural reinforcements can enhance their adhesion to polymer matrices. This work reports the effects of different treatments on the fibre–matrix compatibility in terms of surface energy and mechanical properties of composites. The composites were compounded with two kinds of flax fibres (natural flax and flax pulp) and polypropylene. The applied treatments were maleic anhydride (MA), maleic anhydride-polypropylene copolymer (MAPP) and vinyl trimethoxy silane (VTMO). The treatment effects on the fibres have been characterised by Infrared Spectroscopy. Two techniques have been used to determine the surface energy values: the Dynamic Contact Angle method for the long flax fibres and the Capillary Rise method for the irregular pulps. The use of different methods involves a small discordance in the wettability values. Nevertheless, the three treatments reduce the polar component of the surface energy of the fibre. Composites containing MAPP-treated did the highest mechanical properties, whilst the MA and VTMO-treated fibre gave similar values to that for the untreated ones.

Enhancing water repellence and mechanical properties of gelatin films by tannin addition

In order to reduce pollution caused by traditional non-biodegradable plastic films, renewable raw materials from plants and wastes of meat industries have been employed in this work. A hydrolysable chestnut-tree tannin was used for gelatin modification. Films of gelatin and gelatin-tannin were obtained by casting at room conditions. Transition temperatures of both gelatin and gelatin-tannin systems were determined by differential scanning calorimetry (DSC). Glass transition temperatures of modified gelatin occurred at higher temperatures than for neat gelatin. Enthalpy and temperature of helix-coil transition decreased when tannin content increased due to variations in the helical structure of gelatin as a consequence of tannin presence in agreement with X-ray analysis. Mechanical and thermal behaviour varied as a function of the content of tannin, showing optimum values for films modified with 10 wt% tannin. The transparency of films was maintained after modification with tannin. Solubility and swelling tests of the films revealed that the presence of tannin reduced the water affinity of gelatin.

Mechanical behavior of wood/polypropylene composites: Effects of fibre treatments and ageing processes

Practical applications of wood based composites are limited by the hydrophilic nature of cellulose, which causes dispersion and moisture absorption problems. This work rests on the effects of wood treatment on the mechanical properties of composites made with wood flour and polypropylene (WF/PP). Two different fiber treatments (mercerization with a 10% NaOH solution and functionalization with Epolene E-43 MAPP copolymer), their combined action, and two compounding methods have been studied. In addition, the investigation includes the analysis of the mechanical behavior of composites after exposure to variable conditions of temperature and moisture.

The effect of stoichiometry and thermal history during cure on structure and properties of epoxy networks

Abstract An investigation was carried out into the effect of amine/epoxy stoichiometry and thermal history during cure on physical and mechanical properties of epoxy networks. The formulation studied consisted of a diglycidyl ether of bisphenol A epoxy resin and 4,4′-diaminodiphenyl sulfone curing agent. The experimental matrix was based upon three amine/epoxy ratios and seven different thermal histories during cure. Techniques used included dynamic mechanical and fracture analysis, and Fourier transform infra-red (FTi.r.) spectroscopy. The highest glass transition temperature (Tg) was observed in the stoichiometric formulation and the lowest in the epoxy-rich mixture. For a given stoichiometry, the value of Tg, infinity was not a function of thermal history during cure except, interestingly, in the case when the initial temperature was 180°C. The highest rubbery state modulus and the lowest average molecular weight between crosslinks were also found in the stoichiometric formulation. Our findings were rationalized in terms of the varying degrees of crosslinking in different networks. The opposite trend was observed in the glassy state at 20°C, where the lowest flexular modulus belonged to the stoichiometric formulation. An explanation for those results was offered in terms of the free volume concept. FTi.r. analysis established clearly the existence of residual epoxy groups in all formulations, even after post-cure. Etherification reaction between epoxy and hydroxyl groups takes place during post-cure, but a complete conversion of epoxy groups cannot be attained owing to the topological constraints within the three-dimensional network in the later stages of cure. This finding is of particular significance in mechanistic kinetic models based upon the absolute value of epoxy concentration at all stages of cure.

Bioinspired antimicrobial and biocompatible bacterial cellulose membranes obtained by surface functionalization with aminoalkyl groups.

There has been a great deal of interest in the use of nanostructured bacterial cellulose membranes for biomedical applications, including tissue implants, wound healing, and drug delivery. However, as bacterial cellulose does not intrinsically present antimicrobial properties, in the present study, antimicrobial bacterial cellulose membranes were obtained by chemical grafting of aminoalkyl groups onto the surface of its nanofibrillar network. This approach intends to mimic intrinsic antimicrobial properties of chitosan. Interestingly, these novel grafted bacterial cellulose membranes (BC-NH2) are simultaneously lethal against S. aureus and E. coli and nontoxic to human adipose-derived mesenchymal stem cells (ADSCs) and thus may be useful for biomedical applications. In addition to these biological properties, the bioactive nanostructured BC-NH2 membranes also present improved mechanical and thermal properties.

Bacterial cellulose produced by a new acid-resistant strain of Gluconacetobacter genus.

A bacterial strain isolated from the fermentation of Colombian homemade vinegar, Gluconacetobacter medellensis, was investigated as a new source of bacterial cellulose (BC). The BC produced from substrate media consisting of various carbon sources at different pH and incubation times was quantified. Hestrin-Schramm (HS) medium modified with glucose led to the highest BC yields followed by sucrose and fructose. Interestingly, the microorganisms are highly tolerant to low pH: an optimum yield of 4.5 g/L was achieved at pH 3.5, which is generally too low for other bacterial species to function. The cellulose microfibrils produced by the new strain were characterized by scanning and transmission electron microscopy, infrared spectroscopy X-ray diffraction and elemental analysis. The morphological, structural and chemical characteristics of the cellulose produced are similar to those expected for BC.

Effect of alkaline and autohydrolysis processes on the purity of obtained hemicelluloses from corn stalks

A study of the potential of autohydrolysis and alkaline extraction processes from corn stalks was performed for high purity hemicellulose extraction. The influence of process parameters on the purity of obtained hemicelluloses was analyzed. An experimental design was developed for the autohydrolysis treatments to determine the optimal conditions to solubilize the hemicelluloses with lowest content in contaminants. On the other hand, alkaline extraction, including raw material pretreatment (dewaxing and delignification step) was carried out analyzing the effectiveness of this processes for maximum pure hemicellulose recovery. The maximum yield (54% of the raw material hemicelluloses) and the best physicochemical properties (highest hemicellulose content free of lignin) were obtained with these pretreatments in alkaline extraction. Moreover, the effect of lignin removal by sulfuric acid from the autohydrolysis liquors before hemicellulose precipitation was studied. This purification step has allowed to obtain lignin-free autohydrolysis hemicellulose but with the presence of sulfur as predominant contaminant.

Kinetic and rheological studies of an unsaturated polyester cured with different catalyst amounts

An unsaturated polyester resin was cured with different catalyst contents. The crosslinking process was followed by static and dynamic viscosimetry and by differential scanning calorimetry (d.s.c.). Activation energies were determined from the gel times obtained by viscosimetry and from the variation in the maximum exotherm temperature obtained by d.s.c. at different heating rates. The existence of a threshold in the amount of catalyst to use is shown by analysis of the gel times. The effect of catalyst content on the glass transition temperatures of the different mixtures has been analysed taking into account the crosslink density, measured by means of the rubber modulus obtained by dynamic mechanical tests.

Morphology and thermal behavior of dicyanate ester-polyetherimide semi-IPNS cured at different conditions

A high-temperature thermosetting bisphenol-A dicyanate, BADCy was modified with polyetherimide, PEI, at various compositions. Phase separation and rheokinetics through curing were studied by optical microscopy, dynamic and isothermal differential scanning calorimetry, and rheological measurements. The PEI phase separated at the early stages of curing, well before gelation, and did not affect the polycyclotrimerization kinetics. The phase structure and thermal properties of the final network were investigated as a function of the PEI content and cure temperature. For this purpose, dynamic mechanical analysis, scanning electron microscopy studies, and thermogravimetrical analysis were carried out. The morphological changes were interpreted in terms of a spinodal decomposition mechanism in the composition range studied.

Functional patterns obtained by nanoimprinting lithography and subsequent growth of polymer brushes

In this work the growth of polymer brushes was combined with nanoimprint lithography (NIL) in order to obtain new functional nanopatterns. First, a functional thermoplastic methacrylic copolymer poly(methyl methacrylate-co-2-bromoisobutyryl-oxy-ethyl methacrylate) was synthesized. This copolymer was successfully patterned by NIL using a silicon stamp at 160 °C and 60 bar. Next, hydrophilic polymer brushes based on poly(3-sulfopropylmethacrylate) and hydrophobic polymer brushes based on a poly(fluorinated methacrylate) were grown on the imprinted surfaces. The surface properties of the patterned polymer were accordingly modified and, as a consequence, the water contact angle was modified from 80.3° to 32.5° in the case of the hydrophilic brushes and to 118.1° in the case of the hydrophobic brushes. As an application we demonstrated the use of hydrophobic polymer brushes in order to modify the surface of polymeric stamps for NIL with self-demoulding properties.